that's poor logic, of course you'd look at materials from other countries to contrast local materials. you cant say 'why didnt the british consult siam on penang?' when they did talk to siam years later on the entirety of kedah.

the best he could say is that kedah was a sovereign kingdom that was a vassal of siam, and argue the significance of tribute. tribute can be a recognition of the other's sovereignty over you, simply protection money, or in the unique system of china, a form of government trade (chinese emperor re-gifted more than what they receive, so kingdoms sent tributes often because its profitable).